Let your reader be a fly on the wall
Write what you know. It's one of the most common pieces of advice that writers are given. And no, it's not just meant to save you time on research. That's important too. Write what you know doesn't necessarily mean write your life story (though for some people it does, and those stories can be quite compelling). Authors write about all kinds of things that have never happened to them personally, but that doesn't mean they aren't writing with an authenticity that keeps the reader believing they could have. Write what you know simply means don't stray too far from yourself when you're writing. That thing everyone talks about called voice? That's actually just you. That's the authenticity you can bring to a story by putting a piece of yourself in the writing. The one thing you have as a writer that nobody else has is your unique perspective based on your lifetime of experiences. That's your advantage and it should be a part of everything you write. That's writing what you know.
Regardless of how fantastical your plot may be, you want your reader to feel like a fly on a very real wall. You want them to feel like they're not just reading a story, they're stepping into a rich, colorful, fully-developed world and are spying on it for a while, if not stepping right into the shoes of your protagonist. I'm not a vampire, or an astronaut, or a twelve-year-old boy, and simply outlining what those beings are like isn't enough to grab my attention. What does is the magic that mixes our world and the other--the voice that connects "I know nothing about this" with "But I'd like to learn more" and "There's something about this that I can relate to". If an author doesn't put a piece of their own humanity into their story, doesn't "write what they know", it'll show and the story will be weaker for it. The plot will be there, but the life behind it will fall flat, lacking in the kind of compelling voice that engages us to keep turning the pages. There's nothing wrong with being inspired by other authors, but make sure your writing always comes from a place that's authentically you. Giving your story a real voice is the best thing you can do.